1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to arrangements for retaining an accurate indication of the position of an elevator car in an elevator system, notwithstanding an interruption in the associated electrical power supply.
2. Description of the Prior Art
A floor selector for an elevator car which utilizes a mechanical model of the elevator system is driven by the elevator car via a mechanical link. Thus, an interruption of the electrical power supply does not affect the accuracy of the floor selector. It continues to indicate the correct location of the elevator car in the building.
While this electromechanical type of floor selector provides excellent results, it is being replaced by solid-state floor selectors which are more accurate and easier to maintain. U.S. Pat. No. 3,750,850 , which is assigned to the same assignee of the present application, discloses a floor selector of the solid-state type. In a solid-state floor selector, the position of the elevator car is stored in memory elements, such as a binary counter, with car movement detectors updating the count of the counter to accurately indicate the address or location of the elevator car in the building. The address stored in the counter is compared with the addresses of the floors and the locations of calls for elevator service to create signals for operating and stopping the elevator car at the correct floors.
A disadvantage of the solid-state floor selector is the fact that the conventional solid-state memory devices are volatile. Removal or interruption of the electrical power supply causes the memory devices to lose the information stored therein. Thus, when electrical power returns, following an interruption, the floor selector has "lost" the car. The floor selector no longer knows where the elevator car is located in the building. Latching relays for retaining the car address at the time of power interruption do not account for car movement after power interruption. For example, a car moving at 500 feet per minute at the time of power interruption, will slide for up to 3 seconds before coming to a stop. Batteries for powering the car position circuits of the floor selector are costly, unreliable, and they present maintenance and replacement problems. The hereinbefore mentioned U.S. Pat. No. 3,750,850 resets the floor selector after power returns by causing the car to travel to the lower terminal where the address of the lower terminal is loaded into the car position counter. It would be desirable to be able to reset a solid-state floor selector, however, without the necessity of sending the car to a terminal floor, and without utilizing batteries to energize the car position circuits during a power interruption.